1. Field of the Invention
The present invention relates to the detection of defects in ferrous tubes and, more particularly, but not the way of limitation, to a method and apparatus that employs small area magnetic transducers to sense non-linking parallel flux leakage coupled with digital signal processing to discretely locate and accurately measure axially oriented thickness deviations in the homogeneous body wall of ferrous oil field tubular products.
2. Description of the Related Art
Several types of ferrous tubular products are used in a typical oil field production well system. A production well system can require tubular casing, production tubing, continuous length coiled tubing, sucker rod and a drill string. These ferrous oil field tubular products must be inspected after manufacturing and periodically during use to determine if defects or flaws exist within the homogeneous material.
A variety of defects can exist in ferrous oil field tubular products and have been previously located or detected using several techniques and methods. Localized, discrete defects such as pits, cracks, pin holes and other small, localized defects are detected by inducing a longitudinally oriented constant DC magnetic field into a ferrous tubular product while monitoring for perpendicular magnetic flux leakage with some form of magnetic sensing device, oriented parallel to the surface of the ferrous tube, such as a Hall Effect sensor, induction coil or magneto-resistor. One such system is described in U.S. Pat. No. 5,671,155, which issued Sep. 23, 1997 to EDENS. However, longer, larger, axially extending defects which are typically greater than one inch in length, such as sucker rod wear, erosion/corrosion and co-rod wear, have typically been harder to detect or totally undetectable with perpendicular magnetic flux leakage prior art methods. Often, these types of defects occur at such a gradual rate that little or no perpendicular magnetic flux leakage is generated for detection.
Axially extending defects have been detected with prior art such as the magnetic rotating pole method, the rotating gamma radiation instrument, the electrically rotated magnetic eddy current technique and the saturated magnetic field induction method. Although prior art methods have provided some means of detecting axially extending defects in ferrous oil field tubular products, they suffer from several disadvantages including the requirement of movement, insensitivity to less severe defects, less than one hundred percent coverage of the ferrous tube under inspection, unreasonable cost and size, inaccuracies that yield erroneous results and safety hazard issues.
U.S. Pat. No. 4,555,665, issued Nov. 26, 1985, and U.S. Pat. No. 4,611,170, issued Sep. 9, 1986, both to STANLEY, disclose an apparatus to measure the average wall thickness of ferrous tubing. This is accomplished by integrating an induced voltage in an induction pickup coil, which is induced by changes in the total saturating magnetic field applied to the ferrous tubing by a DC magnetic field generating coil. However, the average wall thickness apparatus does not utilize magnetic flux leakage to obtain the average wall thickness, as stated within the above mentioned patents. This fact would be obvious to one skilled in the art because magnetic flux leakage is not readily detectable away from the surface of a ferrous tubing product since flux leakage decays at an exponential rate through air. This technique also requires movement of the apparatus to induce a sufficiently large voltage change on the induction pickup coil to be measurable, and therefore the reason why integration is used to create a sum of the detected voltages over time. Moreover, an average wall thickness measurement is much less desirable than a discrete, pinpoint measurement.
Prior art gamma radiation systems fail to detect axially extending defects because of the typical eighteen inch helix which is generated from the rotation of the radioactive source around the circumference of the ferrous tubular product as the tubular product is passed through the detection system. A sucker rod wear that is less than eighteen inches in length can easily be missed as the radioactive tool spins around the problem area. Also, since the radioactive beam typically passes through both walls of the ferrous tube to reach the photo-multiplier, which turns the detected radiation into electrical voltage, the result is actually an average thickness of the two points of measurement on the ferrous tube walls. Another detractor of this method is the radioactive source. Much cost and precaution must be taken when handling and operating these older inspection systems, only to yield inadequate results.
The rotating pole magnetic method used in prior art has seen limited use in the past decade due to the immense size and cost of the apparatus required to magnetize typical ferrous oil field tubular products. A typical apparatus to sufficiently magnetize a ferrous tube can weigh in excess of 3000 pounds and is extremely costly to manufacture, therefore making it largely undesirable to implement. Also, this technique requires that the magnetic field be transferred from one pole of the magnet, through the air, into the ferrous tube under inspection, back through the air and into the opposite pole of the magnet as the device spins around the ferrous tube. The spinning of the poles creates a circular, transverse field which, in turn, generates perpendicular flux leakage from longitudinally oriented defects. The ferrous tube acts as a core and means of transmission for the magnetic field. Those of ordinary skill in the art know that magnetic fields decay exponentially through air and is the reason why this magnetizing device must be so large. It is necessary to generate as much as 20,000 gauss to properly saturate a ferrous tubular product with this technique.
U.S. Pat. No. 4,492,115, issued Jan. 8, 1985, and U.S. Pat. No. 4,636,727, issued Jan. 13, 1987, both to KAHIL; U.S. Pat. No. 4,710,712, issued Dec. 1, 1987 to BRADFIELD; and U.S. Pat. No. 4,792,756, issued Dec. 20, 1988 to LAM all disclose an apparatus that detects axially extending defects in ferrous oil field tubing products using an electrically rotated magnetic eddy current method. These apparatuses induce a magnetic AC eddy current around the circumference of the ferrous tubing, which requires a separate set of wire windings to be placed around the circumference of the tube in addition to the windings that are used to induce a constant DC magnetic field into the tubing. These extra windings must be wound with a sine and cosine configuration, to detect thickness variations. Although the apparatuses do not spin, the magnetic AC eddy current field is rotated electrically. One set of sender windings generates a magnetic AC eddy current field in the ferrous tube while a second set of receiver windings provides an AC voltage by the induction method as the AC eddy current propagates through the ferrous tube body wall. The sender and receiver windings are situated as pairs in four quadrants around the circumference of the ferrous tubing, are activated in pairs, and are ninety degrees apart from one another. The measured AC voltage from the receiver winding is monitored for phase and amplitude change and compared to values that have been determined through laboratory testing to extrapolate into the amount of missing material in the body wall of the ferrous tube in the ninety-degree span. However, this technique yields erroneous results if more than one defect exists within the ninety degree span, which is very common in ferrous oil field tubular products. Having multiple defects in the ninety-degree span changes the phase and amplitude of the magnetic AC eddy current more than one time, which yields incorrect results because the phase and amplitude change is no longer relative to the laboratory tested values. Also, these apparatuses require both a constant DC magnetic field and a magnetic AC eddy current field, which must be electrically rotated, to obtain results. The DC magnetic field is used to help obtain better saturation into the ferrous tube body wall because AC eddy currents decay through both air and ferrous material at an exponential rate, similar to ultrasonic waves.
Accordingly, a method and apparatus that can detect axially extending defects in ferrous oil field tubular products, but does not have the requirement of movement; does not require an extremely large, expensive apparatus to rotate a DC magnetic field; and does not require an additional magnetic AC eddy current field would be a vast improvement over the aforementioned systems. Also, an apparatus that provides one hundred percent coverage of the ferrous tubing with the ability to accurately measure multiple defects within the same cross sectional portion of the tubing without erroneous results which utilizes a testing method that poses no hazardous working conditions would be a great improvement in the art of ferrous tubular inspection.